Why is there dew on the grass in the morning?

KohakuOn a morning walk, you sometimes find the grass all damp, don't you? Where did that water come from?

The answer in 30 seconds

The answer lies in the night.

Dew forms on clear nights with little wind, when the surface of the grass loses heat to the sky and cools down, and the water vapor in the air next to it turns into water droplets. Dew does not suddenly appear in the morning; it forms during the night, and we find it in the morning. It forms less easily on cloudy or windy nights.

Round drops of dew lined up on a long, thin blade of grass — Dew forms during the night, and we find it still there in the morning.Photo: Yinan Chen / Public Domain / Wikimedia Commons

Dew comes from water vapor in the air

Air contains water vapor that we cannot see. As air cools, the water vapor starts to turn into water droplets at a certain temperature. This temperature is called the dew point. When the surface of the grass becomes colder than the dew point, the water vapor next to it turns into droplets and clings to the surface. This is dew.

Grass cools on clear nights

At night there is no sunlight, so the ground and the grass lose heat to the sky and cool down. Grass leaves and flower petals lose heat more easily than the air, so at night they become colder than the air around them. On clear nights with no clouds, heat escapes even better, so the grass gets colder still.

Figure: Nights when dew forms easily, and when it doesn't

KohakuSo the clearer the night, the colder the grass gets.

Where and on which nights dew forms

Dew forms easily on the surfaces of things that heat from the ground does not reach well; grass and car roofs are good examples. Also, strong wind stirs up the cooled air, so dew forms most on calm nights with little wind, when an area of high pressure covers the region. Moist air near the ground is another condition.

Dew forms during the night

In other words, dew does not suddenly appear once morning comes. It forms little by little during the night, and we find what is still there in the morning. It can also form in the evening. When the temperature drops below 0℃, dew freezes or turns into frost.

Figure: Dew forms during the night

Want to know more?

In some areas, up to about 0.5 mm of dew can form in a single night, making it an important source of water for plants and animals in dry regions.
Drops lined up evenly along the edges or tips of leaves may not be dew. They can be "guttation": water a plant has taken up through its roots and pushed out through tiny holes in its leaves.
Though it is rarely seen, dew on grass can catch the sunlight and make a band of colors like a rainbow.

KohakuDew forms during the night on the cooled surface of the grass. Morning is simply the time when we find it.

Quick quiz: test yourself

When does the dew on morning grass form?

Show the answer

When the morning sun hits it
On clear nights with little wind
At noon after it rains

On clear nights with little wind On clear nights with little wind, grass loses heat to the sky and cools, and water vapor nearby turns into droplets. It forms at night and is found in the morning.

Sources

DewMet Office
dewEncyclopaedia Britannica
dew - Glossary of MeteorologyAmerican Meteorological Society
Guttation: A Pressure Relief for PlantsUniversity of Missouri Integrated Pest Management

This article was written after checking the sources above as of 2026-10-03. Explanations may change as research progresses. Photo credits are shown below each photo.